1. Field of the Invention
The present invention relates generally to an extendable bit, and more particularly to an extendable bit used in a method in which an air hammer excavates the ground with a casing pipe, preventing the collapse of geological strata.
2. Description of the Related Art
Japanese Patent Publication No. 56-24758 discloses an extendable bit used in the above-mentioned excavation method.
This extendable bit consists of a bit body and bit teeth provided at the point of the bit body. The bit teeth are formed to have substantially the same diameter as the bit body, and the bit teeth are supported by guide grooves formed on a slope at the point of the bit so that the bit teeth can be slidable. A position regulating member regulates the upper and lower positions of the bit teeth. When the bit teeth are pressed against the bottom of an excavation hole, they slide along the guide grooves diagonally upward so that they can be extended. If the bit body rotates in such a state, the bit teeth rotate about an axis of the bit body, and thereby a larger diameter hole than the bit body can be excavated.
The above-mentioned conventional extendable bit, however, has an disadvantage of being weak because there are formed a number of notches such as guide grooves at the center of the bit body.
The conventional extendable bit has another disadvantage in that the excavation cannot be efficiently performed and a large amount of energy must be used in order to perform the task.